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Discussion Starter #1
What is the second engine coolant cap and small reservoir on top of the engine center right?
What does it do?
Thanks for indulging my ignorance.
 

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Coolant expansion tank.
when coolant heats up, it expands, so it needs to go somewhere, when the car cools down, it goes back.
 

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What, it doesnt do that in other cars?

Pretty sure thats exactly what expansion tanks are for, and all cars have em. They’re not sealed/pressurized.

I think it’s just a way to fill from the highest point. The expansion tank doesn’t need to be the highest, and isn't. The top tank, whatever we cal it is sealed, and in fact, if it is filled with coolant, there is no space for expansion
 

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Discussion Starter #5
No, the expansion tank is to the left of the main radiator cap. This upper one is a pressure cap as well.
 

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That makes the entire coolant system an "expansion tank" since it is all under pressure. An Expansion tank will be VTA and allow the coolant expansion to be recovered... That tank is part of the system. I'd say it's more of a thermal siphoning tank for the turbo.
 

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That makes the entire coolant system an "expansion tank" since it is all under pressure. An Expansion tank will be VTA and allow the coolant expansion to be recovered... That tank is part of the system.
Yes, and regardless of what parts people call it (and w'ell have to too) if function is not an expansion tank. If you fill it with incomprehensible coolant,there is no space for expansion - Additionally and there is a lower pressure path for that expanding fluid to go, out the radiator cap into the actual expansion tank.


I'd say it's more of a thermal siphoning tank for the turbo.
You've written this before, please explain. I don't even know the full coolant system, but I'd be willing to bet all necessary and engineered coolant flow is due to pump pressure.
 

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Yes, and regardless of what parts people call it (and w'ell have to too) if function is not an expansion tank. If you fill it with incomprehensible coolant,there is no space for expansion - Additionally and there is a lower pressure path for that expanding fluid to go, out the radiator cap into the actual expansion tank.




You've written this before, please explain. I don't even know the full coolant system, but I'd be willing to bet all necessary and engineered coolant flow is due to pump pressure.
Hot coolant from the turbo goes up to the reservoir, which is placed at the highest point, which draws in "cooler" coolant from the bottom.
 

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I object to the word draw. Hose goes from the block "forced" under pump pressure to the turbo. The pressure pushes the coolant along its path to lower lower pressure at the top of the radiator.

If you disconnect that hose from the turbo and block the turbo fitting to keep it from draining, and hold the hose up, even add an extension hose so that it become s the highest point, and run your engine, coolant will be pumped out - without the turbo or its heat.
 

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The FSM calls it a Coolant filler tank. It's common on newer vehicles to use a remote tank to fill the cooling system. One nice feature of this at least on a Subaru is it means you don't need an Airlift to properly purge the cooling system of air.

One thing to note also on a Subaru is the cap on the remote tank and radiator are different pressures. Accidentally mixing them up will prevent the cooling system from working properly. The cap on the remote tank is set to a lower pressure because it bleeds coolant off into the expansion tank.

I really think they left the second cap to make it easy to purge air from the radiator. I have a friend that owns a shop specializing in Subaru's. Every time he fills the coolant on a WRX without the second cap he needs to carefully wedge a pocket screwdriver between the upper radiator hose and nipple until coolant comes out. Otherwise there's a bubble of air forever trapped at the top of the radiator.

 

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This is almost as bas as some social media posts, lol


The upper reservoir is just a high point fill. That's all. It's the point where expanded fluid leaves and when contracting, coolant enters (from the expansion tank next to the radiator). DO NOT switch the caps! The only reason there is a cap ON the radiator is to speed up filling the system. Many Subarus only have the upper reservoir and NO cap on the radiator.


here is the cap info...





http://photos.killerbmotorsport.com/i.ashx?gallery=345327&mid=68426754&mt=Photo&standardsize=1600x1200
 

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This is almost as bas as some social media posts, lol


The upper reservoir is just a high point fill. That's all. It's the point where expanded fluid leaves and when contracting, coolant enters (from the expansion tank next to the radiator). DO NOT switch the caps! The only reason there is a cap ON the radiator is to speed up filling the system. Many Subarus only have the upper reservoir and NO cap on the radiator.


here is the cap info...


Image Link



http://photos.killerbmotorsport.com/i.ashx?gallery=345327&mid=68426754&mt=Photo&standardsize=1600x1200
I like the way you think... ;)
 

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I object to the word draw. Hose goes from the block "forced" under pump pressure to the turbo. The pressure pushed the coolant along its path to lower lower pressure at the top of the radaitor.
The pump is off when the car is off. Which is what I'm really referring to... Sorry, I thought this was common knowledge (and hence, "siphoning"). When the car is on, it's just part of the pressurized system, along with the rest of the system, and is the highest point.

The radiator cap helps filling the system when drained since it is connected to the large inlet/outlet hoses, but the process can be completely done from the top reservoir.
 

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Thermal syphoning happens anytime the in/out coolant ports are not on the horizontal plane. Garrett and other OEMs precisely spec out what the angle should be. For example, the Garrett GTX ideal angle is 20° from horizontal. When done correctly, coolant will continue to flow (at a snails pace) per the laws on convection currents. This function does not require a reservoir above the turbo height.
 

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Correct, though likely not as effective.

You could be entirely correct on this:

* The block below has a lot of heat capacity. It will provide hot/warm coolant for a while.
* The tank above could allow it's coolant to cool quickly providing a nice differential for a whilr

But it bothers me that the tank is in hot air above the engine when you stop . . . . and the time that flow would be most beneficial is right after shutdown. You'd have to observe . . .

Even if it doesn't work, it could have been the original intent.
 
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